Within the food industry, e.g. the dairy industry, it is often of vital importance to have knowledge about the characteristics of various food products, such as their chemical composition and their associated concentrations. One method of measuring these characteristics utilizes a spectrometer. The spectrometer typically measures the intensity of electromagnetic radiation which is transmitted through, or reflected by, a sample as a function of a collection of wavenumbers or wavelengths, or a wavenumber band, comprised in a particular region of the electromagnetic spectrum, such as the infrared part of the spectrum. The band wavenumber position may be used to identify the content in a sample by means of its chemical structure.
The food products to be analyzed may be of liquid, solid or gaseous form and held in a sample cuvette for analysis. For example, liquid food products may be milk, wine, cream or yoghurt. Moreover, solid food products may be cheese, meat, grain, etc. If the sample is of liquid or gaseous form, the sample is typically kept in a flow-through cuvette during measurements.
A spectrometer comprises a lot of sensitive optical elements, whereby it needs to undergo a careful calibration procedure before it can be put to use. The sensitive optical elements are exposed to wear and tear of various types, e.g. induced by operation of the spectrometer, well as changing operational conditions, such as changes in the surrounding atmospheric conditions. More specifically, the intensity and the wavelength need to be calibrated before a reliable measurement can be initiated. The measured intensities of two different spectrometers typically differ when analyzing the same sample due to their different background spectra. For example, each background spectrum may comprise information about the electromagnetic source, optical parts in the spectrometer as well as intrinsic detector properties. Thus, the background spectrum needs to be subtracted from the measured spectrum in order to obtain a spectrum which is independent of the particular spectrometer used.
A problem with these spectrometers is that each of them needs to be calibrated, which may be a tedious and time consuming task. Fortunately, methods for standardizing spectrometers have been developed in order to solve this problem. In U.S. Pat. No. 5,933,792 there is disclosed a method for standardizing a spectrometer which generates an optical spectrum from a sample. According to the method, one or several optical spectra of a standardization sample, such as a mixture of water and propanol, are obtained by a spectrometer to be standardized, whereby each optical spectrum shows a characteristic pattern in a predetermined frequency range. These characteristic patterns are then compared to reference patterns which constitute the desired standard responses from the standardization sample. Thereafter, a set of standardizing parameters, describing the transition of the generated characteristic patterns of the spectrometer to be standardized to the reference patterns, are determined and stored. Thereby, according to the method as disclosed in U.S. Pat. No. 5,933,792, calibrations may be transferred between different spectrometers at will. A calibrated spectrometer typically has to be recalibrated at regular time intervals.
However, during operation, the cuvette is often degraded by the sample comprised therein, which causes the calibration to become unstable over time.